Evidence from patients with Alzheimer's disease suggest that the basal forebrain cholinergic system plays an important role in memory. In support of this proposal, we have found impaired visual recognition memory in monkeys with lesions to the major nuclei of this system. We have found further that recognition memory in normal monkeys can be improved by the cholinesterase inhibitor physostigmine and impaired by the cholinergic muscarinic-receptor blocker scopolamine. In addition, our results indicate that scopolamine acts at a very early stage of memory, preventing information from entering even into an immediate store. Based on previous results indicating that THC may be exerting its effects through an action on the limbic system, we administered this drug to monkeys performing spatial reversal, a task known to be sensitive to hippocampal damage. Doses equal to or even greater than those that impaired recognition memory did not affect performance on this task, though performance did appear to be affected several days after the last dose of THC, raising the possibility of a delayed abstinence effect. In a series of experiments on habit formation, we administered the dopaminergic-neurotoxin MPTP to monkeys, but failed to show learning impairments at doses that did not also impair motor function. Administration of MPTP did make the animals more sensitive to the disruptive effects of scopolamine, however, suggesting that the cholinergic-dopaminergic balance in the neostriatum had been compromised.